For both economic and environmental reasons, silver is recovered from photoemulsions. The photoemulsion may be from exposed and developed film, undeveloped film (green film), or unused emulsion, e.g., emulsion which does not meet specifications. The silver exists in the emulsion primarily as silver halide. The emulsion also comprises gelatin which encapsulates silver halide particulates and generally a latex, such as acrylic latex or styrene/butadiene latex. Photoemulsions are aqueous; water comprising upwards of 80% by weight.
Silver recovery involves reduction of the silver in the silver halide to its zero-valance, metallic state to produce a silver-containing sludge, drying and burning the sludge in a kiln to remove organic components, and successive melting/purification of the metallic silver content obtained thereby.
The present invention is directed to reduction of the silver halide content by the combination of sodium borohydride (NaBH.sub.4) (SBH) and reducing sugar, which combination achieves a greater recovery of silver than reduction obtainable with either reducing agent alone. Reduction of silver halide using either reducing sugar or SBH alone are known. Reducing sugars are those sugars that give positive oxidation tests in Tollens' or Benedicts' solutions and are generally mono- or di- saccharides capable of reducing copper or silver salts in alkaline solutions. Also included as reducing sugars are polysaccharides which hydrolyze to yield polyhydroxy aldehydes and ketones. Generally, all carbohydrates containing a hemiacetal group or hemiketal group are classified as reducing sugars. Common examples of reducing sugars are sucrose, fructose, and glucose.
Sodium borohydride reduces sugar halide according to the formula: EQU 8AgX+NaBH.sub.4 +2 H.sub.2 O.fwdarw.NaBO.sub.2 +8HX+8Ag.sup.0,
where X is a halide. Thus, one mole of SBH reduces 8 moles of silver halide. SBH in aqueous solution is typically at about 12% by weight in about 40% by weight NaOH (e.g., VenMet.sup.R solution sold by Morton International, Inc.); stoichiometrically, about 8 parts by weight of silver metal are reduced by 1 part by weight of 12% SBH solution.
There are disadvantages with reduction by either reducing sugar alone or sodium borohydride alone which limit silver recovery. While either will reduce a major portion of the silver halide, it is to be appreciated that in silver recovery, each additional percent recovery is significant from economic, environmental and apparatus standpoints. Reduction by reducing sugar is highly pH-dependent, being optimal at a pH of about 12. However, due to hydrogen halide being produced by the silver reduction, it is difficult to maintain such a high pH. Consequently, reduction of silver halide by reducing sugar may be less than desired.
SBH efficiently reduces silver halide that is freed from emulsion; however, it is found that SBH does not reduce silver halide which remains emulsified, e.g., in gelatin. Applicants have found, for example, that a silver halide reduction with SBH may produce a reduced sludge which titrates to a very low silver halide content, but that the sludge, when roasted, takes a heavy corrosive toll on the kiln. This is attributed to residual silver halide content which remains encapsulated in the latex or gelatin.
One way to break the emulsion is to dry the emulsion to a powder and then reconstitute the silver halide in water prior to adding the silver halide. Because a photoemulsion is largely water, i.e., upward of about 80% by weight, substantial energy is required to produce the powder. In reconstituting, it is typical to use large volumes of water so as to provide a viscosity consistent with standard mixing apparatus. The present invention is directed to providing enhanced silver halide reduction from photoemulsion which need be neither dried nor reconstituted and to which a minimal amount of water is added.